The bit rate of a digital file has to do with how many bits the file uses up in a given interval of time. Typically, the higher the bit rate at which music is encoded, the better the playback sounds. For example, a rate of 128 kbps (kilobits per second) is popular in online music downloading since it offers a good compromise between sound quality and download time. Bit rates lower than 128 kbps are generally not suitable for CD (Compact Disc) or hard drive-based devices. Very low bit rates (below 64 kbps) compress voice fairly well and are well suited for online voice chat or streaming radio.
Two other descriptors of digital file sound quality are bit width and sample rate. Most digital music is encoded with a bit width of 16 bits, and a sample rate of 44.1 kHz (CD quality), or lower. Music encoded with a higher bit width and sample rate is considered High Resolution (HR) or High Definition (HD) audio (hereinafter referred to as ‘HD audio’). Since music is typically recorded in a studio at higher bit widths and sample rates than CD quality, HD audio files more closely reproduce the true sound of such music files. Music files recorded in two channels, left and right, are stereo music files. Music files that are recorded in more than two channels are called multi-channel recordings, and a common multi-channel format is the 5.1 HD Surround, which includes six (6) music channels for left, right, center, rear left, rear right, and subwoofer.
Constant Bit Rate (CBR) audio files use up the same amount of data from one moment to the next. A Variable Bit Rate (VBR) audio file uses a lower bit rate in areas of the song that are simpler to compress accurately, and higher bit rates in parts that require more bits to describe accurately. A VBR sound file generally sounds better than a CBR file of the same size. VBR files are somewhat difficult to stream over the Internet since the amount of incoming data constantly changes from one moment to the next.